The present invention relates to cooling systems for motor-vehicle internal combustion engines.
In recent times, car manufacturers have posed an increasing attention in order to obtain an optimal distribution of the temperatures of the engine so as to provide a reduction of the fuel consumption and noxious emissions in the exhaust gases.
More in detail, it is necessary to distinguish the following main problems: the reduction in time for warm-up of the engine after that it has been started at cool temperature; the need of keeping the temperatures on the walls of the combustion chamber and above all on the cylinder walls as higher as possible during this warm-up period; and the need of controlling the engine temperature during normal operation.
There is further the need, above all at partial loads, of keeping the temperature of the engine block relatively high, in order to increase the fluidity of the lubricating oil and decreasing the friction losses, whereas it is necessary to keep the temperature of the engine cylinder head relatively low, in order to avoid detonation at full load. In other words, there is an interest to differentiate the average temperature of the engine block and that of the cylinder head in order to decrease the mechanical losses on one hand and to avoid the risk of detonation on the other hand. Even if at partial loads the engine could stand temperatures of the head comparable with those of the engine block, this condition is anyhow to be avoided since it is not possible nor advisable to cool the engine head during the relatively short time interval which is necessary for coming to a high operating load of the engine. Therefore, the temperature of the cylinder head must be kept substantially constant at any running condition of the engine, while the possibility of varying that of the engine block must be provided.